feat: implement sync operation termination support

[juanxiu]

What does this PR do / why we need it:
This PR implements sync operation termination support for ArgoCD Agent, allowing users to abort running sync operations that were previously unstoppable.

Problem

Currently, when users trigger sync operations on Applications in ArgoCD Agent, there was no way to abort them once started. This is because the .status.operationState.phase field (which indicates "Terminating" state) was not propagated from the Principal to the Agents.

Solution:

• Added a new OperationTerminate event type for signaling sync abort requests
• Principal now detects when .status.operationState.phase changes to "Terminating" and sends this event to the targeted Agent
• Agent receives the event and updates the local Application status to "Terminating", allowing the ArgoCD Application Controller to abort the sync operation

Key Changes:

1. Event System: Added OperationTerminate event type in internal/event/event.go
2. Principal Logic: Enhanced principal/callbacks.go to detect terminating state changes and send events
3. Principal Event Handling: Added OperationTerminate event processing in principal/event.go
4. Agent Processing: Implemented sync termination handler in agent/inbound.go

Which issue(s) this PR fixes:

Fixes #565

How to test changes / Special notes to the reviewer:

Testing Approach

Due to TLS certificate complexity in the full environment, we implemented comprehensive unit tests to validate the core logic:

Unit Tests Added:

1. Principal Callback Tests (principal/callbacks_operation_terminate_test.go):

   • 8 scenarios covering all phase transitions (Running/Succeeded/Failed → Terminating)
   • Edge cases: nil operation states, already terminating states
   • Validates shouldSendOperationTerminateEvent function logic

2. Principal Event Processing Tests (principal/event_operation_terminate_test.go):

   • Agent mode validation (Managed vs Autonomous)
   • Event type handling verification
   • Mock-based dependency isolation

3. Agent Inbound Tests (agent/inbound_operation_terminate_test.go):

   • Success/failure scenarios with mocks
   • Operation termination handling verification
   • Mode-specific event processing

4. Integration Tests (test_sync_termination_integration_test.go):

   • Complete sync termination workflow validation
   • Event flow and structure verification
   • Agent mode behavior validation

Checklist

• Documentation update is required by this PR (and has been updated) OR no documentation update is required.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 14.70588% with 58 lines in your changes missing coverage. Please review.
✅ Project coverage is 45.45%. Comparing base (f6e8e9b) to head (ee02bc8).

Files with missing lines	Patch %	Lines
agent/inbound.go	0.00%	43 Missing ⚠️
principal/event.go	0.00%	10 Missing ⚠️
principal/callbacks.go	66.66%	4 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #595      +/-   ##
==========================================
- Coverage   45.62%   45.45%   -0.18%     
==========================================
  Files          90       90              
  Lines       12021    12089      +68     
==========================================
+ Hits         5485     5495      +10     
- Misses       6091     6148      +57     
- Partials      445      446       +1     

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